1. Field of the Invention
The invention relates to a method for conditioning of moulding sand, wherein the conditioning takes place at least in part in a vacuum. The present invention furthermore relates to a device for implementing the method.
2. Background Art
The conditioning of sand for producing casting moulds is intended to produce the correct mixture ratio of grain sizes as well as the ratio of amounts of quartz sand, binder, powdered coal and used and new sand, to homogenise the mixture and thus to largely coat the grain with the binder, to regulate the correct moisture content, to remove unwanted components, to regulate the correct temperature of the moulding sand, and lastly to transport the ready, conditioned sand to where it is to be used.
In general, the used sand has a raised temperature of, for example, between 100° C. and 140° C. As sand temperatures over approximately 50° C. can present significant problems for moulding machines, and at too high temperatures variations in moisture in the finished sand occur because of uncontrollable evaporation losses on the way between the mixer and moulding installation, in this case the sand has to be cooled.
For this, fluidised bed coolers are mostly used that the sand continuously passes through by means of oscillatory movements of a sieve grate. The cooling principle is that water sprayed onto the sand with nozzles evaporates, and the evaporation enthalpy necessary for this is drawn from the sand as sensible heat. The disadvantage of the method is, however, that very large amounts of air are necessary for transporting away the water vapour occurring, which in turn requires additional energy use.
In DE 29 52 403 C2 therefore, an alternative cooling method was developed. In accordance with this, simultaneous conditioning and cooling of clay bonded foundry moulding sands takes place in a vacuum mixer. The individual components are firstly placed in the mixer.
After a brief pre-homogenisation the temperature and moisture of the mixture is determined and the required amount of water added. Lastly, during the conditioning process, the pressure in the mixer is gradually lowered. As soon as the pressure corresponding to the vapour pressure curve of water is reached, the water in the sand begins to boil and draws the evaporation heat necessary for this from the sand. In this way extremely effective cooling is obtained inexpensively.
The cooler of DE 29 52 403 C2 is only usefully employed when used sand is returned to the mixer at a temperature such that cooling is necessary.
After longer breaks in operation, for example at weekends or due to a breakdown, or where there is low thermal stressing of the moulding sand, for example, because of varying casting temperatures or varying cooling times for the cast shape, the low temperatures of the used sand temperatures do not necessitate cooling. In such cases the moulding sand mixer is operated without the vacuum. Even when used sand is expensively conditioned without a vacuum, it nevertheless differs from moulding sand that has been conditioned using a vacuum.
It is highly desirably in all foundries that the characteristics of the sand are kept as constant as possible in order to obtain consistent quality of the products of the moulding installation.